Crystallization phenomena in diesel fuels and in light oils are getting worse for two reasons:
(1) the mesh size of filter elements is becoming smaller and smaller, in the sixties it was about 30 microns, it has dropped over the years to about 10 microns and recently 5-micron meshes have become available; filters with a mesh of 2 microns may become available in the near future, and this will lead to filter elements clogging as soon as the fuel reaches its cloud point; and
(2) there are increasing quantities of large paraffin molecules in diesel fuels and light oils in addition to the small paraffin molecules which have always existed therein. Small paraffin molecules contain only a few carbon atoms and precipitate at around 0.degree. C. in the form of microcrystals which are 1 to 2 microns in size, this phenomenon is known as the "cloud" point. In very cold weather the microcrystals coagulate into lumps and then become treacly. Heretofore pumps have continued to be able to draw-in such fuel even at very low temperatures because of the sufficiently large pore size of the filters through which the fuel passes, i.e. about 5 microns. It is also important to observe that such small crystals remelt at a temperature only slightly higher than the temperature at which they appear.
Since the beginning of the oil crisis, the oil companies have been leaving an increasing quantity of large paraffin molecules in diesel fuels and light oils for reasons of refining economy. These large molecules respond quite differently to the cold. They remain supercooled for one or more weeks even in very cold weather. They can thus continue to be pumped together with lumps and treacly coagulations of smaller paraffin molecules as described above. However, they can be caused to crystallize rapidly, i.e. in a few minutes or even a few seconds as a result of a shock, for example the shock of being projected against a filter element. These large molecule crystals may be 80 to 200 microns in size. Worse, once they have appeared they are difficult to melt; at 25.degree. C., 20% have melted; at 45.degree. C. 80% have melted, and the remainder melt in the range 60.degree. C. to 65.degree. C. The greater the number of carbon atoms in a molecule, the higher its melting temperature.
Although numerous additives are always present and serve to modify various physical aspects of the unfavorable phenomena described above, they are by no means certain of avoiding the unwanted practical effect of filters becoming clogged.
Effective liquefying apparatus must therefore include a heating surface at close to 70.degree. C. Unfortunately, in order to avoid loss of power and also engine damage, it is important to avoid heating diesel fuel or light oil to a temperature of greater than 40.degree. C. (after the initial pre-heat period prior to starting the engine). Temperature regulation of the hot surface of the liquefying resistances must therefore be dissociated from temperature regulation of the overall flow of diesel fuel or light oil. It should also be observed, with respect to motor vehicles, that vehicles are not presently mass produced with a sufficiently powerful supply of electricity to be capable of raising the average temperature of the flow of fuel to above the melting point of large paraffin crystals. The term "liquefy" is being used in the present application to cover the melting of paraffin crystals rather than just keeping the temperature high enough to retard their crystallization.